I have two bettas living in a divided tank. The divider was seethrough, I have made it unseethrough. During the time they could see each other they flared a lot. Especially Billie. Billie's operculum are now inflamed.
Is the inflammation painful or would it be causing any probelms for him? How can I fix it if it is causing pain or probelms? Or do I need to let it uninflame by itself? If it is purely cosmetic, it doesnt worry me.

Are you sure his operculum is inflamed? It doesn't seem likely to me that he could flare "too much".
Usually gill-related issues occur due to water chemistry problems, such as high ammonia. Do you change the tank often enough? Does it have a filter?

All he really needs to heal is consistently clean water, but if you're afraid of infection you could dose him with aquarium salt. You'll need to move him to his own heated 1 gallon container though. You'll use 1 tsp. aquarium salt per gallon, with daily 100% water changes to prevent salt buildup. It probably does hurt him a little, just keep him calm so he's not stretching anything out too much.

I am curious as to why you think they are inflamed? If the gill cover is red then that is a sign of water chemistry/quality problem.. too much ammonia or nitrates, etc.

What signs is he showing? Don't treat with salts just yet until you know what it is he has for sure - no need to stress them out unnecessarily even more so.

And they should calm down after a few days to a week of being placed together in a divided tank.. most of these domesticated bettas prefer to see another fish/person as they are quite social. They won't flare themselves to death, nor to harm.. will flare as they please then turn away when they want a rest. So don't worry if they do ever see each other again.. just make sure they continue to be active and eat healthy - if one or both stops being active and such, you may want to let them see each other once more so they can have some stimulation.

AyalaCookieJar- this morning I changed it.
LittleFish2012- It may not be inflamed but there is something wrong with it.
There could possibly be something wrong with the water. But it started in clean water when he saw the other fish and continually flared. Although they had changed tanks and soon after the crappy gravel began to cloud his water. I have gotten new good gravel now though so that could of been the cause.
I don't have a filter as of yet. It is in the post and will hopefully be here soon.
BabyStarz- thanks for the advice.
Myates- I believe it is because of over flaring as it started when he began to see the other betta. Although it could of been a coincidence.
Although I have minimal betta knowledge, there operculum could get sore I suppose from over use. Like when you eat a lot of very chewy lollies or chew gum for ages your jaw gets sore.
His operculum are not red. Just now they have discovered some stops in the divider and have started flaring again.
I took some photos but they are quite crappy and I don't know how to load them up, so I'll try to describe.
His operculum are not going right back into place and there is a small whitish patch on the top of his head the sides of his face are tinged are bit whiter. Now that I think about it it's probably a disease. But it hasn't spread to the others fish so...
Any way thanks for your help! Everyone.